The present invention relates to a Pockels cell and a switch with a Pockels cell. By Pockels cell is meant an electro-optical cell able to change the polarisation of a beam crossing through it via the application of an electric field to the cell. Said cell may be cut in birefractive crystal whose cristallographic axes are deviated by the presence of an electric field parallel to the optical axis of the crystal. This warrants the name xe2x80x9cPockels cell with longitudinal fieldxe2x80x9d.
The Pockels cell of the invention is used in particular to produce switches or optical modulators.
Advantage of the invention may be taken in particular for the fabrication of regenerative amplifiers.
Regenerative amplifiers use a trigger switch in two states. In a first state, photons are trapped and amplified in a laser cavity, while in the second state the photons are removed from the cavity.
To achieve the switch function, systems are used with which the polarisation of the laser beam can be switched by 90xc2x0 by means of return excursion in a Pockels cell controlled by an electric voltage.
Conventionally a KDP (potassium-dihydrogen-phosphate) crystal bar is used having a length of 2 to 3 cm, whose ends are provided with electrodes.
To carry out optical switching in two states, it is necessary to successively apply two independent voltages of high potential (for example 4000 V) to each electrode in order to set up or cancel a polarising electric field in the cell. The quick changeover from one voltage to the other requires costly and complex electronic switching equipment. It proves to be difficult to provide electronic switching equipment which achieves both fast set-up of a high voltage on a terminal and fast cancellation of this voltage.
To avoid this difficulty, the single Pockels cell switch is replaced by a switch with two Pockels cells. Said switch is described in greater detail below with reference to appended FIG. 1.
FIG. 1 shows a section of the switch 10 comprising two Pockels cells 12a and 12b aligned along an optical axis 14 between a first window 16 and a second window 18. The Pockels cells are enclosed in a sealed case 20 to provide protection from humidity. The switch also comprises polarisation or analysis means arranged in front of the windows that are not shown in the figures for reasons of clarity.
The Pockels cells 12a and 12b are each formed of a cylindrical KDP bar. At the ends of the KDP bars, ring-shaped electrodes 26a, 26b, 28a, 28b are formed. A voltage applied between the electrodes of a Pockels cell 12a, 12b, leads to creating an electric field, parallel to the optical axis 14, in the KDP bar.
The voltage applied to the electrodes is piloted by an electronic circuit 30 having three polarisation terminals 32, 34 and 36.
The first polarisation terminal 32 is connected to a first electrode 26a of the first Pockels cell 12a. The second terminal 34 is connected both to a second electrode 28a of the first Pockels cell 12a and to a first electrode 26b of the second Pockels cell 12b. 
Finally, the third terminal 36 is connected to a second electrode 28b of the second Pockels cell.
The functioning of the switch is explained with reference to FIG. 2.
FIG. 2 shows in graph form the voltages V32 and V36 applied to the first and third terminals of the electronic circuit 30 in relation to time. It also shows the difference in voltage V32xe2x88x92V36.
Voltages V32 and V36 are expressed in relation to a mass potential which is applied in constant manner to the second terminal 34 of electronic circuit 30.
As from an initial switching time denoted ton, a high voltage V0 is applied to the first terminal 32. A beam crossing through the first Pockels cell subsequently undergoes a phase difference of xcex/4 (xcex/2 if the beam makes a return excursion within the cell). The second non-polarised cell does not cause any phase difference at this time.
A rise time is defined as being the time needed to set up the voltage V32 of the first terminal at a high voltage value VO, as from the time this terminal is at mass voltage. This rise time is in the order of 3 ns.
As mentioned previously, for electronic circuit construction-related reasons, it is difficult to achieve switching of a high voltage V0 to the mass potential in a time that is as short as the rise time mentioned above. The rise time to set up the high voltage is therefore shorter than the time needed for its cancellation.
Therefore, to interrupt the polarisation effect of the first Pockels cell quickly, the high voltage V0 is also applied to the third terminal 36, that is to say on the second Pockels cell.
The electric field set up in the second Pockels cell 12b is then opposite in direction to the field existing in the first Pockels cell 12a. The light beam undergoes a phase difference of xe2x88x92xcex/4 in the second cell.
The beam crossing through the switch therefore undergoes a first phase difference of xe2x88x92xcex/4 in the first cell 12a and a second phase difference of xe2x88x92xcex/4 in the second cell 12b, and therefore emerges from the device with zero phase difference. The performance of the switch is then the same as in the state in which none of the cells are polarised. Fast optical switching can thus be obtained.
In FIG. 2, the time at which the voltage V36 of the third terminal is established at value V0 is indicated by toff. It may be noted that the rise time of this voltage is also in the order of 3 ns.
Finally, the optical switch is open before time ton and after time toff. During time toffxe2x88x92ton, called the laser impulse creation time, V32xe2x88x92V35=V0 and the switch is closed since the total phase difference undergone by the light beam which effects a return excursion in the switch is xcex/2 (xcex/4+xcex/4).
For additional information on the fabrication of the electronic circuit 30 of the device in FIG. 1, reference may be made to French patent application nxc2x0 96 10006 dated Jun. 8, 1996.
The fabrication of an electro-optical device such as described above encounters a certain number of difficulties among which particular mention may be made of the alignment of the Pockels cells.
Indeed it is, necessary to perfectly align the two Pockels cells 12a, 12b along the optical axis 14.
In addition, the use of two identical cells to increase switching speeds multiplies the production cost of the device.
Finally, when the beam crosses through the device, it meets the four faces of the Pockels cells and therefore undergoes fairly considerable attenuation of its intensity.
The present invention sets out to provide a dual Pockels cell, that can be used for example in an optical switch such as previously described and which does not encounter the difficulties described above.
One purpose, in particular, is to provide said cell with which it is possible to fabricate a switch or an electro-optical modulator without taking any steps to align the optical cells along one same optical axis.
A further purpose is to provide a cheap electro-optical device in which the attenuation of a light beam may be reduced in comparison with known devices.
To achieve these purposes, the subject matter of the invention is more precisely a dual Pockels cell comprising:
a single bar in birefractive material extending continuously between a first and a second end,
a first and a second electrode, arranged at the ends of the bar,
a third electrode, arranged on the bar between the first and second electrodes.
The electrodes are, for example, ring-shaped electrodes surrounding the bar around its circumference.
Via the third electrode, two electric fields, which may or may not be of opposite direction, may be set up, whether simultaneously or not, between the third electrode and the first and second electrodes.
Therefore, the bar of birefractive electro-optical material may be used in the same way as two Pockels cells aligned along one same axis.
However, a beam of light crossing through the cell of the invention only meets two end faces and is therefore only weakly attenuated.
Also, the cost of fabricating a dual cell of the invention is substantially the same as that for an ordinary single cell with two electrodes.
According to one particularly advantageous embodiment, the third electrode may be positioned substantially equidistant from said first and second electrodes.
Through this characteristic, an electric field of same intensity may be formed in the cell either side of the third electrode, through the application of a voltage of the same value to the first and second end electrodes.
Other characteristics and advantages of the invention will emerge more clearly from the following description which refers to the figures of the appended drawings. This description is given solely for illustration purposes and is non-restrictive.